Records

This proposal tracks the specification for the C# 9 records feature, as agreed to by the C# language design team.

The syntax for a record is as follows:

record_declaration
    : attributes? class_modifier* 'partial'? 'record' identifier type_parameter_list?
      parameter_list? record_base? type_parameter_constraints_clause* record_body
    ;

record_base
    : ':' class_type argument_list?
    | ':' interface_type_list
    | ':' class_type argument_list? ',' interface_type_list
    ;

record_body
    : '{' class_member_declaration* '}' ';'?
    | ';'
    ;

Record types are reference types, similar to a class declaration. It is an error for a record to provide a record_base argument_list if the record_declaration does not contain a parameter_list. At most one partial type declaration of a partial record may provide a parameter_list.

Record parameters cannot use ref, out or this modifiers (but in and params are allowed).

Inheritance

Records cannot inherit from classes, unless the class is object, and classes cannot inherit from records. Records can inherit from other records.

Members of a record type

In addition to the members declared in the record body, a record type has additional synthesized members. Members are synthesized unless a member with a "matching" signature is declared in the record body or an accessible concrete non-virtual member with a "matching" signature is inherited. A matching member prevents the compiler from generating that member, not any other synthesized members. Two members are considered matching if they have the same signature or would be considered "hiding" in an inheritance scenario. It is an error for a member of a record to be named "Clone". It is an error for an instance field of a record to have an unsafe type.

The synthesized members are as follows:

Equality members

If the record is derived from object, the record type includes a synthesized readonly property equivalent to a property declared as follows:

Type EqualityContract { get; }

The property is private if the record type is sealed. Otherwise, the property is virtual and protected. The property can be declared explicitly. It is an error if the explicit declaration does not match the expected signature or accessibility, or if the explicit declaration doesn't allow overriding it in a derived type and the record type is not sealed.

If the record type is derived from a base record type Base, the record type includes a synthesized readonly property equivalent to a property declared as follows:

protected override Type EqualityContract { get; }

The property can be declared explicitly. It is an error if the explicit declaration does not match the expected signature or accessibility, or if the explicit declaration doesn't allow overriding it in a derived type and the record type is not sealed. It is an error if either synthesized, or explicitly declared property doesn't override a property with this signature in the record type Base (for example, if the property is missing in the Base, or sealed, or not virtual, etc.). The synthesized property returns typeof(R) where R is the record type.

The record type implements System.IEquatable<R> and includes a synthesized strongly-typed overload of Equals(R? other) where R is the record type. The method is public, and the method is virtual unless the record type is sealed. The method can be declared explicitly. It is an error if the explicit declaration does not match the expected signature or accessibility, or the explicit declaration doesn't allow overriding it in a derived type and the record type is not sealed.

If Equals(R? other) is user-defined (not synthesized) but GetHashCode is not, a warning is produced.

public virtual bool Equals(R? other);

The synthesized Equals(R?) returns true if and only if each of the following are true:

• other is not null, and
• For each instance field fieldN in the record type that is not inherited, the value of System.Collections.Generic.EqualityComparer<TN>.Default.Equals(fieldN, other.fieldN) where TN is the field type, and
• If there is a base record type, the value of base.Equals(other) (a non-virtual call to public virtual bool Equals(Base? other)); otherwise the value of EqualityContract == other.EqualityContract.

The record type includes synthesized == and != operators equivalent to operators declared as follows:

public static bool operator==(R? left, R? right)
    => (object)left == right || (left?.Equals(right) ?? false);
public static bool operator!=(R? left, R? right)
    => !(left == right);

The Equals method called by the == operator is the Equals(R? other) method specified above. The != operator delegates to the == operator. It is an error if the operators are declared explicitly.

If the record type is derived from a base record type Base, the record type includes a synthesized override equivalent to a method declared as follows:

public sealed override bool Equals(Base? other);

It is an error if the override is declared explicitly. It is an error if the method doesn't override a method with same signature in record type Base (for example, if the method is missing in the Base, or sealed, or not virtual, etc.). The synthesized override returns Equals((object?)other).

The record type includes a synthesized override equivalent to a method declared as follows:

public override bool Equals(object? obj);

It is an error if the override is declared explicitly. It is an error if the method doesn't override object.Equals(object? obj) (for example, due to shadowing in intermediate base types, etc.). The synthesized override returns Equals(other as R) where R is the record type.

The record type includes a synthesized override equivalent to a method declared as follows:

public override int GetHashCode();

The method can be declared explicitly. It is an error if the explicit declaration doesn't allow overriding it in a derived type and the record type is not sealed. It is an error if either synthesized, or explicitly declared method doesn't override object.GetHashCode() (for example, due to shadowing in intermediate base types, etc.).

A warning is reported if one of Equals(R?) and GetHashCode() is explicitly declared but the other method is not explicit.

The synthesized override of GetHashCode() returns an int result of combining the following values:

• For each instance field fieldN in the record type that is not inherited, the value of System.Collections.Generic.EqualityComparer<TN>.Default.GetHashCode(fieldN) where TN is the field type, and
• If there is a base record type, the value of base.GetHashCode(); otherwise the value of System.Collections.Generic.EqualityComparer<System.Type>.Default.GetHashCode(EqualityContract).

For example, consider the following record types:

record R1(T1 P1);
record R2(T1 P1, T2 P2) : R1(P1);
record R3(T1 P1, T2 P2, T3 P3) : R2(P1, P2);

For those record types, the synthesized equality members would be something like:

class R1 : IEquatable<R1>
{
    public T1 P1 { get; init; }
    protected virtual Type EqualityContract => typeof(R1);
    public override bool Equals(object? obj) => Equals(obj as R1);
    public virtual bool Equals(R1? other)
    {
        return !(other is null) &&
            EqualityContract == other.EqualityContract &&
            EqualityComparer<T1>.Default.Equals(P1, other.P1);
    }
    public static bool operator==(R1? left, R1? right)
        => (object)left == right || (left?.Equals(right) ?? false);
    public static bool operator!=(R1? left, R1? right)
        => !(left == right);
    public override int GetHashCode()
    {
        return HashCode.Combine(EqualityComparer<Type>.Default.GetHashCode(EqualityContract),
            EqualityComparer<T1>.Default.GetHashCode(P1));
    }
}

class R2 : R1, IEquatable<R2>
{
    public T2 P2 { get; init; }
    protected override Type EqualityContract => typeof(R2);
    public override bool Equals(object? obj) => Equals(obj as R2);
    public sealed override bool Equals(R1? other) => Equals((object?)other);
    public virtual bool Equals(R2? other)
    {
        return base.Equals((R1?)other) &&
            EqualityComparer<T2>.Default.Equals(P2, other.P2);
    }
    public static bool operator==(R2? left, R2? right)
        => (object)left == right || (left?.Equals(right) ?? false);
    public static bool operator!=(R2? left, R2? right)
        => !(left == right);
    public override int GetHashCode()
    {
        return HashCode.Combine(base.GetHashCode(),
            EqualityComparer<T2>.Default.GetHashCode(P2));
    }
}

class R3 : R2, IEquatable<R3>
{
    public T3 P3 { get; init; }
    protected override Type EqualityContract => typeof(R3);
    public override bool Equals(object? obj) => Equals(obj as R3);
    public sealed override bool Equals(R2? other) => Equals((object?)other);
    public virtual bool Equals(R3? other)
    {
        return base.Equals((R2?)other) &&
            EqualityComparer<T3>.Default.Equals(P3, other.P3);
    }
    public static bool operator==(R3? left, R3? right)
        => (object)left == right || (left?.Equals(right) ?? false);
    public static bool operator!=(R3? left, R3? right)
        => !(left == right);
    public override int GetHashCode()
    {
        return HashCode.Combine(base.GetHashCode(),
            EqualityComparer<T3>.Default.GetHashCode(P3));
    }
}

Copy and Clone members

A record type contains two copying members:

• A constructor taking a single argument of the record type. It is referred to as a "copy constructor".
• A synthesized public parameterless instance "clone" method with a compiler-reserved name

The purpose of the copy constructor is to copy the state from the parameter to the new instance being created. This constructor doesn't run any instance field/property initializers present in the record declaration. If the constructor is not explicitly declared, a constructor will be synthesized by the compiler. If the record is sealed, the constructor will be private, otherwise it will be protected. An explicitly declared copy constructor must be either public or protected, unless the record is sealed. The first thing the constructor must do, is to call a copy constructor of the base, or a parameter-less object constructor if the record inherits from object. An error is reported if a user-defined copy constructor uses an implicit or explicit constructor initializer that doesn't fulfill this requirement. After a base copy constructor is invoked, a synthesized copy constructor copies values for all instance fields implicitly or explicitly declared within the record type. The sole presence of a copy constructor, whether explicit or implicit, doesn't prevent an automatic addition of a default instance constructor.

If a virtual "clone" method is present in the base record, the synthesized "clone" method overrides it and the return type of the method is the current containing type if the "covariant returns" feature is supported and the override return type otherwise. An error is produced if the base record clone method is sealed. If a virtual "clone" method is not present in the base record, the return type of the clone method is the containing type and the method is virtual, unless the record is sealed or abstract. If the containing record is abstract, the synthesized clone method is also abstract. If the "clone" method is not abstract, it returns the result of a call to a copy constructor.

Printing members: PrintMembers and ToString methods

If the record is derived from object, the record includes a synthesized method equivalent to a method declared as follows:

bool PrintMembers(System.Text.StringBuilder builder);

The method is private if the record type is sealed. Otherwise, the method is virtual and protected.

The method:

1. calls the method System.Runtime.CompilerServices.RuntimeHelpers.EnsureSufficientExecutionStack() if the method is present and the record has printable members.
2. for each of the record's printable members (non-static public field and readable property members), appends that member's name followed by " = " followed by the member's value separated with ", ",
3. return true if the record has printable members.

For a member that has a value type, we will convert its value to a string representation using the most efficient method available to the target platform. At present that means calling ToString before passing to StringBuilder.Append.

If the record type is derived from a base record Base, the record includes a synthesized override equivalent to a method declared as follows:

protected override bool PrintMembers(StringBuilder builder);

If the record has no printable members, the method calls the base PrintMembers method with one argument (its builder parameter) and returns the result.

Otherwise, the method:

1. calls the base PrintMembers method with one argument (its builder parameter),
2. if the PrintMembers method returned true, append ", " to the builder,
3. for each of the record's printable members, appends that member's name followed by " = " followed by the member's value: this.member (or this.member.ToString() for value types), separated with ", ",
4. return true.

The PrintMembers method can be declared explicitly. It is an error if the explicit declaration does not match the expected signature or accessibility, or if the explicit declaration doesn't allow overriding it in a derived type and the record type is not sealed.

The record includes a synthesized method equivalent to a method declared as follows:

public override string ToString();

The method can be declared explicitly. It is an error if the explicit declaration does not match the expected signature or accessibility, or if the explicit declaration doesn't allow overriding it in a derived type and the record type is not sealed. It is an error if either synthesized, or explicitly declared method doesn't override object.ToString() (for example, due to shadowing in intermediate base types, etc.).

The synthesized method:

1. creates a StringBuilder instance,
2. appends the record name to the builder, followed by " { ",
3. invokes the record's PrintMembers method giving it the builder, followed by " " if it returned true,
4. appends "}",
5. returns the builder's contents with builder.ToString().

For example, consider the following record types:

record R1(T1 P1);
record R2(T1 P1, T2 P2, T3 P3) : R1(P1);

For those record types, the synthesized printing members would be something like:

class R1 : IEquatable<R1>
{
    public T1 P1 { get; init; }
    
    protected virtual bool PrintMembers(StringBuilder builder)
    {
        builder.Append(nameof(P1));
        builder.Append(" = ");
        builder.Append(this.P1); // or builder.Append(this.P1.ToString()); if P1 has a value type
        
        return true;
    }
    
    public override string ToString()
    {
        var builder = new StringBuilder();
        builder.Append(nameof(R1));
        builder.Append(" { ");

        if (PrintMembers(builder))
            builder.Append(" ");

        builder.Append("}");
        return builder.ToString();
    }
}

class R2 : R1, IEquatable<R2>
{
    public T2 P2 { get; init; }
    public T3 P3 { get; init; }
    
    protected override bool PrintMembers(StringBuilder builder)
    {
        if (base.PrintMembers(builder))
            builder.Append(", ");
            
        builder.Append(nameof(P2));
        builder.Append(" = ");
        builder.Append(this.P2); // or builder.Append(this.P2); if P2 has a value type
        
        builder.Append(", ");
        
        builder.Append(nameof(P3));
        builder.Append(" = ");
        builder.Append(this.P3); // or builder.Append(this.P3); if P3 has a value type
        
        return true;
    }
    
    public override string ToString()
    {
        var builder = new StringBuilder();
        builder.Append(nameof(R2));
        builder.Append(" { ");

        if (PrintMembers(builder))
            builder.Append(" ");

        builder.Append("}");
        return builder.ToString();
    }
}

Positional record members

In addition to the above members, records with a parameter list ("positional records") synthesize additional members with the same conditions as the members above.

Primary Constructor

A record type has a public constructor whose signature corresponds to the value parameters of the type declaration. This is called the primary constructor for the type, and causes the implicitly declared default class constructor, if present, to be suppressed. It is an error to have a primary constructor and a constructor with the same signature already present in the class.

At runtime the primary constructor

1. executes the instance initializers appearing in the class-body

2. invokes the base class constructor with the arguments provided in the record_base clause, if present

If a record has a primary constructor, any user-defined constructor, except "copy constructor" must have an explicit this constructor initializer.

Parameters of the primary constructor as well as members of the record are in scope within the argument_list of the record_base clause and within initializers of instance fields or properties. Instance members would be an error in these locations (similar to how instance members are in scope in regular constructor initializers today, but an error to use), but the parameters of the primary constructor would be in scope and useable and would shadow members. Static members would also be useable, similar to how base calls and initializers work in ordinary constructors today.

A warning is produced if a parameter of the primary constructor is not read.

Expression variables declared in the argument_list are in scope within the argument_list. The same shadowing rules as within an argument list of a regular constructor initializer apply.

Properties

For each record parameter of a record type declaration there is a corresponding public property member whose name and type are taken from the value parameter declaration.

For a record:

• A public get and init auto-property is created (see separate init accessor specification). An inherited abstract property with matching type is overridden. It is an error if the inherited property does not have public overridable get and init accessors. It is an error if the inherited property is hidden.
  The auto-property is initialized to the value of the corresponding primary constructor parameter. Attributes can be applied to the synthesized auto-property and its backing field by using property: or field: targets for attributes syntactically applied to the corresponding record parameter.

Deconstruct

A positional record with at least one parameter synthesizes a public void-returning instance method called Deconstruct with an out parameter declaration for each parameter of the primary constructor declaration. Each parameter of the Deconstruct method has the same type as the corresponding parameter of the primary constructor declaration. The body of the method assigns to each parameter of the Deconstruct method, the value of the instance property of the same name. The method can be declared explicitly. It is an error if the explicit declaration does not match the expected signature or accessibility, or is static.

The following example shows a positional record R with its compiler synthesized Deconstruct method, along with its usage:

public record R(int P1, string P2 = "xyz")
{
    public void Deconstruct(out int P1, out string P2)
    {
        P1 = this.P1;
        P2 = this.P2;
    }
}

class Program
{
    static void Main()
    {
        R r = new R(12);
        (int p1, string p2) = r;
        Console.WriteLine($"p1: {p1}, p2: {p2}");
    }
}

with expression

A with expression is a new expression using the following syntax.

with_expression
    : switch_expression
    | switch_expression 'with' '{' member_initializer_list? '}'
    ;

member_initializer_list
    : member_initializer (',' member_initializer)*
    ;

member_initializer
    : identifier '=' expression
    ;

A with expression is not permitted as a statement.

A with expression allows for "non-destructive mutation", designed to produce a copy of the receiver expression with modifications in assignments in the member_initializer_list.

A valid with expression has a receiver with a non-void type. The receiver type must be a record.

On the right hand side of the with expression is a member_initializer_list with a sequence of assignments to identifier, which must be an accessible instance field or property of the receiver's type.

First, receiver's "clone" method (specified above) is invoked and its result is converted to the receiver's type. Then, each member_initializer is processed the same way as an assignment to a field or property access of the result of the conversion. Assignments are processed in lexical order.